1. Field of the Invention
The present invention relates to a Liquid Crystal Display (LCD) device, and more particularly, to an LCD device having an Optically Compensated Bend (OCB) mode which prevents an initial high current while supplying a high voltage to a liquid crystal.
2. Description of the Related Art
An LCD device is thin, lightweight, and low in power consumption compared to a Cathode Ray Tube (CRT), and has also less electromagnetic wave emission. Thus, LCD devices have been widely used as displays of portable information devices such as cellular phones, computers, and Personal Digital Assistants (PDAs).
However, an LCD device has different brightness and color according to the angle at which it is observed, and thus has a narrow viewing angle. Various ways of resolving this viewing angle problem have been suggested.
For example, in order to improve the viewing angle range of an LCD device, one technique arranges a prism plate on a light guide panel to improve the straightness of light emitted from a back light, so that the brightness in a vertical direction is improved more than 30%. Also, another technique provides a negative compensation film to improve the viewing angle range.
Furthermore, an In Plane Switching mode has been developed to achieve a wide viewing angle of 160°, which is almost the same as that of a CRT. However, this technique results in a low aperture ratio and thus needs further improvement.
Moreover, in order to improve the viewing angle range, Thin Film Transistor (TFT) driving techniques including an Optically Compensated Bend (OCB) mode, a Polymer Dispersed Liquid Crystal (PDLC) mode, a Deformed Helix Ferro-electric (DHF) mode, and so on, have been suggested.
In particular, the OCB mode has been the focus of considerable research and development efforts because it has a rapid liquid crystal response speed and a wide viewing angle.
An initial orientation state of a liquid crystal arranged between an upper plate electrode and a lower plate electrode is a homogeneous state, and when a predetermined voltage is supplied across the upper and lower plate electrodes, the liquid crystal state changes via a transient splay and an asymmetric splay into a bend state and then operates in an OCB mode.
Liquid crystal molecules in a central portion of a liquid crystal layer are left-and-right symmetrically arranged, and thus a tilt angle is 0° at less than a predetermined voltage and 90° at more than the predetermined voltage. A high voltage is initially supplied, so that the tilt angle of the liquid crystal molecules in the central portion of the liquid crystal layer becomes 90°, and then the supplied voltage is varied to change the tilt angle of liquid crystal molecules not in the central portion of the liquid crystal layer, thereby modulating the polarization of light passing through the liquid crystal layer.
It takes tens of seconds to arrange the tilt angle of the liquid crystal molecules in the central portion from 0° to 90°, and a response time is as fast as 10 μm since there is no back flow and a big bending transformation has a large elastic modulus.
When the OCB mode is in an ON state, conversion from the transient splay to the asymmetric splay is fast, and conversion from the transient splay to the bend state is relatively fast, but conversion from the asymmetric splay to the bend state is slow.
When the OCB mode is in an OFF state, conversion to the homogeneous state is slow but conversion from the transient splay to the homogeneous state or from the asymmetric splay to the homogeneous state is fast.
As described above, there is a problem in that a predetermined time (hereinafter, a “transient time”) passes before the bend orientation for the OCB mode is obtained. Therefore, an LCD device often supplies an initial high voltage to a common electrode of the liquid crystal in order to reduce the transient time in the OCB mode.